1. Field of the Invention
The present invention relates to a printing apparatus that employs suction to hold a print medium on a platen and performs printing.
2. Description of the Related Art
For printers, copiers, facsimile machines, scanners and multifunctional machines and systems, printing apparatuses, for which print heads are provided, are employed that, based on image data, print images on print media such as printing sheets. One commonly used apparatus of this type is an ink jet printing apparatus, which prints images on print media by ejecting ink, through ejection ports in a print head, onto the print media.
In an ink jet printing apparatus, suppressing deformation (also called cockling) of a print medium, such as wrinkling or the formation of wavy edges, that occurs when ink is applied, is required for performing printing in stable. Also, in the print medium, suppressing occurring deformation such as a curling is required. It is because the print medium may touch a print head or a carriage and cause the quality of a printed image to be deteriorated, when a print medium on a platen is deformed.
For example, a method for maintaining a constant distance between a print medium and a print head is proposed in Japanese Patent Laid-Open No. 2009-78571. According to this proposal, suction means is provided for a platen located opposite a print head, and suction is applied to hold a print medium on the platen. According to a printing apparatus described in Japanese Patent Laid-Open No. 2009-78571, printing can be performed while print medium deformation, such as curling or cockling, is suppressed. As the suction means provided for holding a print medium, grooves are formed, in a conveying direction, in a platen whereon a print medium is supported, and suction hole arrangements are prepared in the grooves, so that in use, a negative pressure is applied to the print medium, via the suction holes, so as to hold the print medium on the platen. Further, even when cockling has occurred in the print medium, since the grooves are formed in the platen, the cockled portions of a print medium will enter the grooves. With this arrangement, variations in distance between the print head and the print medium, due to deformation of the print medium by cockling, can be suppressed.
Furthermore, according to the printing apparatus disclosed in Japanese Patent Laid-Open No. 2009-78571, grooves are located between a set of two adjacent ribs, and two types of ribs are provided, comparatively low and comparatively high. Since the printing apparatus is arranged in this manner, when a print medium having relatively low stiffness, i.e., a print medium having low flexural rigidity and high flexibility, is held on a platen by suction, printing can be performed while the print medium is arranged along the grooves. On the other hand, when a print medium having relatively high stiffness, printing can be performed while the print medium is supported on the ribs. Therefore, different types of print media, for which the degree of stiffness varies, can be conveyed while being held on the platen by suction.
However, in the printing apparatus disclosed in Japanese Patent Laid-Open No. 2009-78571, since a print medium is suctioned from the entire area of a platen evenly, in a case that the cockled condition of the print medium is partially changed, there is possibility that the apparatus may not cope appropriately. When printing is performed with a high print duty, there is possibility that the cockling becomes greater by increasing the amount of ink provided for a unit print area. When suction is performed with a high suction pressure to hold a print medium for which larger-scale cockling has even occurred by high-duty printing, on the platen appropriately, it is difficult for a print medium for which printing is performed at a low print duty to be appropriately held on the platen. For printing performed at a low print duty, such as printing of line art, there is a case wherein the scale of cockling is small. Especially, in a case wherein printing at a low print duty is performed for a print medium, such as plain paper having a comparatively low stiffness, or proofing paper, due to suction through suction holes, a part of the print medium might move into a gap between the ribs, even though no cockling of the print medium has occurred. Accordingly, there is a possibility that a print medium will be arranged on a platen in the wavy condition. In this case, the distance between the print medium and the print head varies, depending on the positioning, so that deviations may occur in the landing positions of ink droplets ejected by the print head, and an unevenness may occur in the printing of regular lines, such as hatching.
Furthermore, there is an idea that the suction pressure applied through the suction holes should be reduced in order to avoid printing unevenness. However, when the suction pressure is merely reduced, deformation of a print medium caused by curling or cockling can not be suppressed, and the quality of a printed image may be degraded.